1. Field of the Invention
The subject invention relates to the recording, playback, transmission and other processing of video and other recurring information and, more specifically, to methods and apparatus for compressing the bandwidth of video and other recurring information, and particularly to picture bandwidth compression.
2. Prior Art Statement
As a general statement, information transfer systems may be broadly described in terms of five components: an information source, an information sink, an information transfer channel, and an encoder and a decoder which match the source and the sink, respectively, to the transfer channel. In the case of a transmission system, the transfer channel is a channel through which the information is transmitted. If the information transfer is effected through recording and subsequent playback, the transfer channel includes information recording and playback equipment.
In the special case of cinematography, the information transfer channel includes a motion picture film, and the information sink includes the human observer as the ultimate receiver of the motion picture. As to the technical aspect of cinematography, a chief goal is to impart to the human observer the illusion of motion in the presented picture, ideally in a flicker-free display. In this respect it has been found that the requisite information update generally is significantly higher for the achievement of a flicker-free display than it is for the impartation of the desired illusion of motion.
In recognition of this psychophysical phenomenon, motion pictures are typically displayed at a rate, such as twenty-four frames per second, which is satisfactory for a creation of the desired illusion of motion, but which would be inadequate to avoid flicker in the presented pictures. However, flicker is practically avoided through the use of an agency, such as a mechanical chopper, which breaks each displayed frame into several distinct flashes of luminance for a virtual frame rate equal to the number of flashes per frame multiplied by the number of frames per second. For instance, if the motion picture film is advanced at twenty-four frames per second, then a mechanical chopper may break each frame into three distinct flashes of luminance for a virtual frame rate of seventy-two frames per second, thereby avoiding flicker for practical purposes without incurring the high expense and inconvenience of a film speed of seventy-two frames per second.
Some of the efforts to reduce the requisite bandwidth of video signal transmissions are described in the book PICTURE BANDWIDTH COMPRESSION, edited by Thomas S. Huang and Ohl J. Tretiak (Gordon and Breach, Science Publishers, 1972). Page 11 of that book designates the interlace of TV lines as epitomizing the contribution of visual psychophysics to engineering, and introduces a number of articles dealing with flicker effects and other phenomena concerning the human observer.
In this respect, it is well known that commercial television operates at a rate of approximately sixty fields per second in the NTSC system and at fifty fields per second in the PAL and SECAM systems. These field rates were the result of efforts to provide a flicker-free display in the context of a supply power line frequency of 60 Hz and 50 Hz, respectively.
Most video bandwidth compression approaches have been inspired by the recognition, as in the above mentioned case of cinematography, that the field rate or update requisite to a flicker-free display produces redundancy as far as the generation of an illusion of motion in scenes displayed to the human observer is concerned.
In this respect, reference may be had to U.S. Pat. No. 3,944,728, by Leonard A. Ferrari, issued Mar. 16, 1976 to the subject assignor, and including a description of an advanced skip field technique in a recording and playback environment. The requisite recording channel bandwidth is kept low by recording at a reduced speed only as many fields per second as required for a preservation of the illusion of motion in the resulting image display. Flicker is avoided, on the other hand, by a repeated reproduction of recorded fields at increased recording medium speed.
Another approach, known as inter-frame encoding and described in Part 6 of the above mentioned PICTURE BANDWIDTH COMPRESSION book, pp. 623 et seq., proposes to reduce bandwidth by a frame-to-frame correlation technique which typically confines the updating of information to the occurrence of picture element changes among succeeding fields or frames. A major drawback of such systems, which so far has prevented their use on a commercial scale, is their need for a large storage facility coupled with extensive hardware for a detection and evaluation of interframe picture element changes.
In this respect, reference may be had to the picture point interlace method illustrated on page 634 of the above mentioned PICTURE BANDWIDTH COMPRESSION book, wherein different picture elements are processed in different passes. In particular, that frame-correction coding system transmits motion sequences at lower data rates than used in standard practice by correcting a fixed fraction of the picture sample points during each frame transmission.
In the intra-frame encoding field, described in Part 5 of the above mentioned PICTURE BANDWIDTH COMPRESSION book, the application of Fourier-Hadamard and other transformation techniques to bandwidth compression has been proposed as may be seen from pp. 515 to 573 of that book. Though this approach appeared promising, its commercial application has been faltering.
In this respect, U.S. Pat. No. 3,976,826 by J. M. Fullton, issued Aug. 24, 1976, appears to evince a recognition of the importance of different orders of coefficients in a Hadamard or Fourier transformation context, when proposing that the highest order coefficient relating to basis vector H(0), which gives the amplitude (brightness) of the overall subpicture, might always be selected, and that then the four largest remaining coefficients might be chosen. However, the text and disclosure of that patent are deficient of a useful coefficient selection strategy.
The subsequent U.S. Pat. No. 4,037,248, by Iijima et al, issued July 19, 1977, in effect rejects the orthogonal or Hadamard transformation technique in favor of a special sampling method.